1. Field of the Invention
The present invention generally relates to a moving body controlling device, an intermediate transferring device and an image forming apparatus having the intermediate transferring device.
2. Description of the Related Art
Image forming apparatuses such as copiers, printers and facsimile machines typically have an image forming device including a photoreceptor drum for forming an image, an intermediate transferring device for transferring the image formed by the image forming device to a recording medium and a fixing device for fixing the transferred image.
Further, the image forming apparatuses ordinarily include a moving body controlling device for carrying recording media (which may be ordinarily referred to as a recording paper) into an intermediate transferring device and a fixing device and carrying the recording media out of the intermediate transferring device and the fixing device.
Ordinarily, the intermediate transferring devices are formed by combining an endless belt suspended and rotated by plural rollers and a contacting roller that is in contact with the endless belt.
The intermediate transferring devices transfer toner images transferred from the image forming devices to the belts while holding recording papers by nipping the recording papers with a contacting portion (nipping portion).
Since the intermediate transferring devices are a device for driving a belt, the intermediate transferring devices may be used not only as image forming apparatuses but also as moving body controlling devices for carrying various sheet-like materials. In case of intermediate transferring devices for color image forming apparatuses of electrophotographic systems, belts are used as recording media for primary transfer and have functions of secondary transfer to recording papers. In such intermediate transferring devices, when there are variations in rotational speeds of the belts, deformation or color shift may occur on images transferred to the recording papers. Therefore, it is necessary to accurately control carrying speeds of the intermediate transferring devices, i.e. surface speeds of the belts, to avoid the deformation and the color shift.
A relationship between an intermediate transferring medium of an image forming apparatus and a contacting roller (a secondary transferring roller) is described next. Conventionally, an encoder is installed in a belt driving shaft (a driving roller shaft) as a technique of controlling the intermediate transferring medium. Signals from the encoder is fed back to a first control unit for driving the belt to thereby control the rotational speed of the first motor. Thus, the speed of the intermediate transferring medium can be controlled. Further, there is a conventional technique that a scale is provided on a surface of the intermediate transferring medium to enable a sensor to read the scale. Thus, the speed of the intermediate transferring medium is measured. The measured speed information is used to control the driving speed of the first motor. Resultantly, the speed of the belt can be more accurately controlled.
At this time, the secondary transferring roller in contact with the intermediate transferring medium is led by the intermediate transferring medium or is driven and rotated by a second motor. When the secondary transferring roller is rotated by the second motor, a control target rotational speed of the second motor is determined so that the surface speed of the intermediate transferring medium is the same as the surface speed of the secondary transferring roller.
Patent Document 1 discloses an image forming apparatus having an intermediate transferring device that does not cause deformation and color shift of images when images are copied to recording papers.
In the image forming apparatus, a speed profile of the intermediate transferring medium is measured, a speed profile of a secondary transferring roller in contact with the intermediate transferring medium is controlled to have a profile of speed variation reverse to the profile of speed variation in the intermediate transferring medium. Therefore, it becomes possible to prevent deformation of secondary transferred images on recording papers, which are transferred from the intermediate transferring medium.
In this case, recording papers which undergo the secondary transfer are carried in conformity with the profile of speed variation of the secondary transferring roller. Meanwhile, the speed of the intermediate transferring medium at a nipping portion is set substantially the same as the recording papers. Therefore, a carrying speed of the recording papers measured by a measuring unit and a carrying speed of the recording papers at the nipping portion may differ. In this case, it is rarely problematic if the profile of speed variation of the intermediate transferring medium has a small amplitude or a waveform of an extremely long period. However, if the intermediate transferring medium does not have the small amplitude and the waveform of the extremely long period, slack or stretch may occur. As a result, images may be deformed.
When the secondary transferring roller is driven by a second motor, the outer diameter of the secondary transferring roller may be increased by temperature rise or the like. As a result, the surface speed of the secondary transferring roller may become higher than the surface speed of an intermediate transferring medium. Therefore, there occurs a problem that the rotational speed of the intermediate transferring medium increases because the intermediate transferring medium is led by the secondary transferring roller thereby being forced to be driven. In ordinary brushless motors, the rotational speed is adjusted by a pulse width modulation (PWM) signal and cannot undertake decelerating or reversing control. Therefore, it is not possible to reduce the speed when the rotational speed becomes higher than a target value due to an extraneous effect. Therefore, there is a case where the rotational speed becomes higher than the target value even though an instruction value of PWM for the first motor is minimized to be an instruction value of the rotational speed due to leading by the secondary transferring roller. In this case, it is not possible to control the speed of the intermediate transferring medium to be constant. Thus, there are problems that images are not stably output, and a control unit of an image forming apparatus detects abnormality in the rotational speed to thereby stop the motor. Resultantly, the image forming apparatus may be stopped.
Not only in the intermediate transferring medium, when there are a moving body (a carrying body or a rotating body) and another moving body (another carrying body or rotating body) affecting the rotation of the moving body, the moving body is sometimes lead by the other moving body, and the other moving body including a moving body including a moving body is sometimes lead by the moving body. Then, the problems identical to or similar to those described above may occur.    Patent Document 1: Japanese Laid-Open Patent Application No. 2005-338703